A. Field of the Invention
The present invention relates to an ultrasonic transducer module for an ultrasonic sensor for detection and/or examination of value documents, to an ultrasonic sensor for detection and/or examination of value documents having such an ultrasonic transducer module, and to a method for manufacturing the ultrasonic sensor.
B. Related Art
Value documents are understood in this connection to be sheet- or card-shaped objects that represent for example a monetary value or an authorization and hence should not be manufacturable arbitrarily by unauthorized persons. Hence, they have features that are not simple to produce, in particular to copy, whose presence is an indication of authenticity, i.e. manufacture by an authorized body. Important examples of such value documents are chip cards, coupons, vouchers, checks and in particular bank notes.
Upon the machine processing of such value documents, in particular of bank notes, ultrasound is frequently used. Ultrasound can be used for example for detecting value documents, i.e. for recognizing the passing of a value document upon transport, and/or for examining value documents, for example for recognizing so-called double removals, i.e. incompletely singled value documents of a value-document stack or bundle, or value documents transported in partial overlap, and/or for capturing ultrasonic properties of the value document, for example for ascertaining the state of the value documents. In so doing, ultrasound is first coupled into air by a transducer, is then propagated in the air until impinging on a value document, where applicable, and is then remitted or transmitted by the value document. The ultrasound thus emitted by the value document can then be captured again. Due to the necessary coupling into air and the decrease in coupling efficiency with increasing ultrasonic frequency, the ultrasound employed usually does not have a very high frequency. Ultrasound is hence understood within the scope of the present application to be sound with frequencies between 40 kHz and about 1 GHz.
For generating and receiving such ultrasound there are employed, at least in the area of detecting and/or examining value documents, piezoelectric ultrasonic transducers which convert electrical vibrations into mechanical or sound vibrations employing a piezoelectric element. So as not to impair their vibration upon the generation or reception of the ultrasound, they possess connecting strands, i.e. very flexible connecting wires, which are electroconductively connected to the piezoelectric element only indirectly or directly. Hence, ultrasonic transducers with rigid connecting pins are typically not employed. Common piezoelectric elements have a temperature resistance up to about 80° C. or 90° C., so that they must be treated very gently upon the manufacture of sensors.
The thus described ultrasonic sensors are not simple to manufacture, however, because the connecting strands must be contacted to conducting paths on a circuit board which usually has further circuit elements as well.